It is desirable that a hydraulically-assisted engine valve actuator provide for flexible engine valve operation under a wide band of engine operating conditions. The hydraulically-assisted engine valve actuator should provide for variable valve timing of closing and opening and variable lift as desired in order to achieve the greatest engine efficiencies. Presently, hydraulic fluid is supplied to hydraulically actuated valves through tubes commonly called rails. Valve motion profiles in current hydraulic actuation designs depend on a pre-established constant value of oil pressure at the supply rails because rail pressures cannot be adjusted fast enough to modulate valve profiles. The constant rail pressure values result in constant valve profiles regardless of engine rpm.
Present hydraulic actuation schemes add complexity to the engine design. Some hydraulic actuation designs rely on additional hydraulic supply rails at constant pressure levels. Further, hydraulic actuation that relies on on/off solenoid (spool or poppet) valve operations require engine valve position sensors for reliable timing of the solenoids and for safe operation. The plurality of sensors required, further adds to the engine complexity.
There is a need in providing such valve actuation to do so in as economical a manner as possible. A linear motor provides an excellent source of control actuation for the valve actuator. However, a linear motor is considerably more expensive than a solenoid. A solenoid should be used if its limitations can be accommodated.
The valve actuator should demonstrate simplicity of module. There should be no double dependencies in order to minimize the criticality of certain machining tolerances. The concentricity requirements of the device should be as lenient as possible.
The valve actuator should readily accommodate the extremes of valve lash that occur in a diesel cycle engine. Within several minutes after starting a cold engine, it is not uncommon for a valve to grow 0.020 inch due to increased valve temperature. It is helpful if the valve control module be not directly coupled to the engine valve so that no complexities need be designed into the control module to account for valve lash.